The present invention relates to a protection system for an electric vehicle and a protection method, and more particularly relates to a protection apparatus and a protection method suitable for an electric vehicle having a current sensor, a speed sensor and an accelerator sensor.
In recent years, an electric vehicle of the vector control type has become widely used since an induction motor driving the electric vehicle is driven with a high speed and a high accuracy by an inverter. In general, an electric vehicle comprises an inverter for converting a direct current electric power source of a battery to a variable voltage and variable frequency alternating current power source, a three-phase alternating current motor for driving the vehicle, a current sensor and a speed sensor for detecting the current and the rotating speed of the three-phase alternating current motor, a torque command calculating means for determining a torque command of the three-phase alternating current motor based on a degree of opening of an accelerator, a three-phase alternating current command generating means for generating a three-phase alternating current command to control phase current flowing in the windings of the three-phase alternating current motor based on the torque command and the outputs of the current sensor and the speed sensor, and a signal generating means for generating a signal to be applied to the gate of the inverter based on the three-phase alternating current command and the current flowing in the windings of the three-phase alternating current motor.
In such an electric vehicle, the torque control apparatus is constructed based on a torque command sent from a driver through the degree of opening of the accelerator, the current of the motor detected by the current sensor, and the rotating angular speed of the motor detected by the speed sensor. In order to keep the safety of such a torque control apparatus, a technology is disclosed in Japanese Patent Application Laid-Open No. 3-277101 (1991) where sensors are constructed in a dual system, so that it is possible to switch from the sensor in one system to the auxiliary sensor in the other system when the sensor in one system is failed, and the motor is driven based on the information from the auxiliary sensor.
On the other hand, a control system is disclosed in Japanese Patent Application Laid-Open No. 5-91601 (1993) where the control system switches from speed feedback control to V/f control when the speed sensor is failed.
Since probability of occurrence of failure is finite in any system, in a driving system of an electric vehicle it is necessary to provide a system capable of safely stopping and further driving in any case on the assumption of occurrence of any failure.
When the sensors are constructed in a multi-system as described in Japanese Patent Application Laid-Open No. 3-277101 (1991), the reliability of the sensors themselves is improved. However, the reliability of the overall torque control system is not always improved since number of cables and connecting terminals for the detected signals is increased. Further, there are some cases where the auxiliary sensor may also fail, depending on the cause of the failure, or the switching to the auxiliary sensor may involve danger depending on the method of switching.
In order to perform vector control, an accelerator switch for detecting the degree of opening of the accelerator, a current sensor for detecting the current of the three-phase alternating current motor and a speed sensor for detecting the rotating speed of the three-phase alternating current motor are necessary and indispensable. Therefore, when failure occurs in these sensors, vector control becomes inoperative and the torque of the induction motor becomes abnormal, resulting in a dangerous state in the driving of the vehicle. For example, the vector control is performed with feedback of the output from the speed sensor in order to accurately control the speed and the torque. Therefore, the speed sensor is necessary and indispensable. There is a problem in that when the speed sensor is failed, driving of the electric vehicle is dangerous due to the possibility of an excessively large or an excessively small torque, or a negative torque, and a lack of stable control in voltage and/or current.
On the other hand, according to the invention disclosed in Japanese Patent Application Laid-Open No. 5-91601 (1993), the vehicle can be driven and stopped safely even when the speed sensor is failed. However, in this invention, there is a problem in that control becomes inoperative when the current sensor is failed since the output from the current sensor is used in the driving control when the speed sensor is failed.